This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No.2001-390038 filed on Dec. 21, 2001; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to a semiconductor device and a method for fabricating the same. More specifically, the invention relates to a CMOS field effect transistor using an oxynitride film as a gate insulator film, and a method for fabricating the same.
2. Related Background Art
Semiconductor devices use various active elements for constituting integrated circuits. One of most typical active elements is a MOS field effect transistor (MOSFET). The MOSFET is a semiconductor device wherein the conduction and non-conduction states between a source and a drain are controlled by a voltage applied to a gate electrode which is formed on a semiconductor substrate via an insulator film. As a typical application of the MOSFET, there is a CMOSFET wherein an n-channel MOSFET and a p-channel MOSFET are provided to complimentarily function.
By the way, the thickness of gage oxide films is decreasing with the scale down of transistors. If the thickness of a gate oxide film is 2 nm or less, the increase of electric power consumption during stand-by due to tunnel current, and the increase of dispersion in threshold due to penetration of boron (B) cause problems. The former causes serious problems in n-channel MOSFETs, and the latter causes serious problems in p-channel MOSFETs. In order to solve these problems, there are some cases where an oxynitride film is used as a gate insulator film.
Oxynitride films can have a higher dielectric constant than that of an oxide film, and can have a larger physical thickness than that of an oxide film having the same capacity, so that it is possible to reduce the tunnel current. Since the dielectric constant of the oxynitride film increases as the concentration of nitrogen increases, the tunnel current can be reduced with the increase of the concentration of nitrogen. However, there is a problem in that the current driving force of a transistor deteriorates with the increase of the concentration of nitrogen. Particularly in n-channel MOSFETs, this problem is conspicuous. Therefore, with respect to n-channel MOSFETs, it can be said that the upper limit of the concentration of nitrogen in the gate oxynitride film is determined by an allowable gate tunnel current.
On the other hand, oxynitride films can more greatly suppress the dispersion of boron and the deterioration of characteristics due to penetration of boron than those in oxide films. As the concentration of nitride increases, the boron penetration suppressing effect increases, so that it is possible to reduce the dispersion in threshold. Therefore, with respect to p-channel MOSFETs, it can be said that the lower limit of the concentration of nitrogen in gate oxynitride films is determined by an allowable dispersion in threshold.
As described above, in p-channel MOSFETs, it is required to provide an oxynitride film having a higher concentration of nitrogen in order to prevent penetration of boron. On the other hand, in n-channel MOSFETs, it is required to suppress the concentration of nitrogen in the oxynitride film in order to prevent the deterioration of the current driving force while reducing the tunnel current.
By the way, in semiconductor integrated circuits, the quantity of allowable tunnel current varies with uses, whereas the quantity of allowable penetrating boron, i.e., the allowable dispersion in threshold, is constant independent of uses. Therefore, the upper limit of the concentration of nitrogen in the gate oxynitride film of an n-channel MOSFET may lower than the lower limit of the concentration of nitrogen in the gate oxynitride film of a p-channel MOSFET.
In a conventional process for fabricating an LSI, the gate insulator films of n-channel and p-channel MOSFETs are simultaneously formed, so that both of the gate insulator films must use oxynitride films having the same concentration of nitrogen.
As a result, it is difficult to simultaneously prevent deterioration of the current driving force in the n-channel MOSFET and penetration of boron in the p-channel MOSFET.
According to one embodiment of a semiconductor device according to the present invention, there is provided with a semiconductor device comprising:
a first MOS field effect transistor of an n-type including a first oxynitride film as a first gate insulator film; and
a second MOS field effect transistor of a p-type including a second oxynitride film as a second gate insulator film, the second MOS field effect transistor being disposed adjacent to the first MOS field effect transistor;
wherein a concentration of nitrogen in the first gate insulator film is different form that in the second gate insulator film.
According to one embodiment of a semiconductor device according to the present invention, there is provided with a semiconductor device comprising:
a first MOS field effect transistor of an n-type including a first oxynitride film as a first gate insulator film; and
a second MOS field effect transistor of a p-type including a second oxynitride film as a second gate insulator film, the second MOS field effect transistor being disposed adjacent to the first MOS field effect transistor;
wherein a difference in thicknesses between the first and second gate insulator films is in proportion to a difference between concentrations of nitrogen in the first and second gate insulator films as a result of introduction of different concentrations of nitrogen into oxide films having had the same thickness.
According to one embodiment of a semiconductor device according to the present invention, there is provided with a semiconductor device comprising:
a first MOS field effect transistor of an n-type including a first oxynitride film as a first gate insulator film; and
a second MOS field effect transistor of a p-type including a second oxynitride film as a second gate insulator film, the second MOS field effect transistor being disposed adjacent to the first MOS field effect transistor;
wherein the first and second gate insulator films have different concentrations of nitrogen and different thickness values as a result of introduction of nitrogen at the same surface density into oxide films having had different thickness values.
According to one embodiment of a method for fabricating a semiconductor device according to the present invention, there is provided with a method comprising:
oxidizing a surface of a semiconductor substrate in first and second MOS field effect transistor regions, which are isolated from each other by an element isolating insulator film of the surface portion of the semiconductor substrate, to form a first oxide film;
introducing nitrogen into the first oxide film to form a first oxynitride film having a first concentration of nitrogen;
depositing a first polycrystalline silicon film on a surface of the first oxynitride film;
forming a first mask on the first polycrystalline silicon film in one of the first and second MOS field effect transistor regions;
removing the first polycrystalline silicon film in the other region of the first and second MOS field effect transistor regions by using the first mask;
removing the first mask to use the first polycrystalline silicon film as a mask, which remains in the one of the first and second MOS field effect transistor regions, to remove the first oxynitride film in the other region of the first and second MOS field effect transistor regions;
oxidizing the surface of the semiconductor substrate in the other region of the first and second MOS field effect transistor regions and a surface of the first polycrystalline silicon film remaining in the one of the first and second MOS field effect transistor regions to form a second oxide film on the surface of the semiconductor substrate in the other region of the first and second MOS field effect transistor regions and the surface of the first polycrystalline silicon film remaining in the one of the first and second MOS field effect transistor regions; and
introducing nitrogen into the second oxide film to form a second oxynitride film having a second concentration of nitrogen.
According to one embodiment of a method for fabricating a semiconductor device according to the present invention, there is provided with a method comprising:
oxidizing a surface of a semiconductor substrate in first and second MOS field effect transistor regions, which are isolated from each other by an element isolating insulator film of the surface portion of the semiconductor substrate, to form a first oxide film having a first thickness;
forming a first mask on the first oxide film in one of the first and second MOS field effect transistor regions;
removing the first oxide film in the other region of the first and second MOS field effect transistor regions by using the first mask;
removing the first mask to oxidize the surface of the semiconductor substrate in the other region of the first and second MOS field effect transistor regions to form a second oxide film having a second thickness, which is smaller than the first thickness, on the surface of the semiconductor substrate in the other region of the first and second MOS field effect transistor regions; and
introducing nitrogen into the first and second oxide films to form first and second oxynitride films.